How to Draw a Class Diagram in UML
Class diagrams are the foundation for all other UML structure diagrams. Use this tutorial to understand the basics of how to draw a class diagram of your own in UML.
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How to Create Class Diagrams
Class diagrams can be somewhat complex, so it's important to understand the basics. Try reviewing our class diagram overview, along with this tutorial on what is UML. That way, you'll have a firm grasp on the underlying principles of UML and class diagrams. Once you're prepared with a diagramming program like Lucidchart, it's time to start drawing.
Record Important Components
Class diagrams have a wide variety of use cases. They can help you:
- Demonstrate responsibilities in a system
- Review and create a static view of an application
- Make subsequent charts, including component and deployment diagrams
- Construct the executable code for forward- and reverse-engineering of a system
Unlike some other UML charts, class diagrams have a wide variety of model elements that can be included. Luckily, the creator has the freedom to include just a few necessary sections in each class diagram shape, or create more complex diagrams. Before you proceed, take a look at the application you want to model and record 3 elements: the name, attributes, and methods that correspond to each class. Then make a note of the relationships that exist between classes.
Add Shapes and Symbols
Every class diagram must include the UML class shape. In Lucidchart, you can begin by dragging out this symbol, along with all others, from the toolbox and placing them on the canvas. The standard class symbol has three rows, but you can use a similar shape that omits the bottom two rows.
To add information in each row of the class shape, just click and type. The first row should include only the class name, while the second features the attributes of the class. Finally, the last section lists potential methods or operation the class could take. The text in the bottom two sections is usually aligned to the left and typed with a lowercase first letters; see the images on this page for an example.
Continue to add class shapes until your static application is fully represented. When you're done, it's time to put three crucial concepts into action: scope, visibility, and interaction.
Indicate scope by underlining certain members (the term for attributes or methods in a class). Members can be classified as either instances or classifiers; the latter are often recognized as static. This is when attribute values remain the same in all instances, and method invocation doesn't affect the instance's state. In comparison, instance members have methods that can change the instance's state, and attribute values that may vary between instances. To indicate a classifier, just underline its name; indicate an instance by leaving the text intact.
Demonstrate visibility—also known as access—by placing the correct symbol within the class shape and before the particular attribute or method. Consult this list to find out which symbol is associated with each access level:
- Public (+)
- Private (-)
- Protected (#)
- Package (~)
- Derived (/)
Show your audience the interactions with class and object diagrams. These are the relationships and links that exist between objects. While there are too many to list here, you can see some of the most common interactions on this class diagram definition page. In Lucidchart, you can indicate relationships between objects by drawing a line, then changing the notation style as needed.
Review Your Work
Before you print off your finished diagram, make sure that it's truly complete. Try asking a mentor or your teammates what they think of the class diagram and whether it's missing anything. They should be able to give you feedback that streamlines the diagram.
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